Memory material and method for its manufacture

ABSTRACT

A composition of materials having ferromagnetic, piezoelectric, and electro-optical properties is disclosed. In the preferred embodiment, the composition of materials comprises a first layer of Pb.sub.(1-x-y) Cd x  Si y , a second layer of Se.sub.(1-z) S z , and a third layer of Fe.sub.(1-w) Cr w , where x, y, z and w are values within the ranges of 0.09≦x≦0.11, 0.09≦y≦0.11, 0.09≦x≦0.11 and 0.18≦w≦0.30. Additionally, each of the layers contain at least one of the elements of Ag, Bi, O, and N. A random-accessible, non-volatile memory built using the invented composition of materials is also disclosed. This memory provides for storing two independent bits of binary information in a single memory cell. Each cell comprises two orthogonal address lines formed on the opposite surface of a Si substrate, a composition of materials of the present invention formed over each of the address lines, and an electrode formed over each composition of materials. The data is stored electromagnetically and retrieved as a piezoelectric voltage.

BACKGROUND OF THE INVENTION

Computer technology requires memories having large storage capacity andhigh speed. Typically, in a modern computer, a semiconductor memory isemployed as high-speed primary memory and magnetic disks are used for alarge volume secondary memory.

Prior to the development of semiconductor memories, the high-speedprimary memory was implemented using a magnetic core memory. A magneticcore memory comprises a matrix of ring-shaped ferromagnetic cores. Eachmemory cell of the magnetic core memory includes a ferromagnetic corehaving two or more wires passing through the center of the core and asensing coil installed around the core.

When a current I is applied to a wire that passes through the core, amagnetic field is produced which has a magnetic field strength H whichis a function of the current I. The magnetic field produced by thecurrent causes a permanent magnetization of the core which is measuredby the magnetic induction B. The relationship between B and H hassubstantial hysteresis with the result that a plot of B versus H, whichis known as the magnetization curve or BH loop, is substantially square.

The magnetic induction B in the core has two states, B_(r) and -B_(r),that correspond to the opposite directions of the magnetic field.Accordingly, each core can store a bit of binary data by associating onestate with a "1" and the other state with a "0". Illustratively, +B_(r)may be associated with a binary "1" and -B_(r) with a binary "0".

The binary data is written into a core memory cell by applyingappropriate currents to the wires. If the total current passing throughthe core is greater than a critical current I_(c), the magneticinduction of the core changes from -B_(r) to +B_(r). Similarly, if thecurrent is less than -I_(c), the magnetic induction switches from +B_(r)to -B_(r). Advantageously, in an array of magnetic cores, switching isperformed as the result of the coincidence of signals on two or morewires. Thus, if the magnetic induction initially has the value of -B_(r)corresponding to a "0", a binary "1" is stored by applying a currentI>I_(c) /2 to each of the two wires, so that the total current passingthrough the core is greater than +I_(c) which causes the magneticinduction to change to +B_(r).

The data stored in the core is retrieved by sensing the voltage acrossthe coil induced by switching between the two magnetic states describedabove. The polarity of the induced voltage indicates the magnetic stateof the core prior to switching.

Although the magnetic core memory described above is random accessibleand non-volatile, such memory is large, consumes a large amount ofpower, operates at a slow speed and can not be manufactured to have ahigh storage density. To overcome these problems, magnetic thin filmmemory devices have been developed. A magnetic thin film memory consistsof a strip of ferromagnetic thin film, two or more wires for writingdata formed on the film and a coil around the film for reading data.

In the thin film memory, the magnetic moment M of the film representsthe stored information. The magnetic moment M is oriented primarily inthe plane of the film, and has two discrete orientations or states M and-M that represent binary "1" and "0". To store a bit of binary data,currents are applied to the wires formed on the thin film. Thesecurrents induce a magnetic field that is sufficient for changing thedirection of the magnetic moment M. The stored information is retrievedby applying currents to the wires and measuring the induced voltage inthe coil. As in magnetic core memory, the currents are typicallyselected such that a single current has insufficient amplitude toreverse the magnetic moment of the film so that at least two coincidentcurrents are required for storing data.

There are significant drawbacks associated with magnetic thin filmmemory technology. First, thin film devices have an open magnetic fluxstructure and therefore the BH loop is smeared by a self-demagnetizingeffect. To reduce this effect, the film is typically fabricated as arectangle whose length is much greater than its width. Since the inducedvoltage in the coil around the film is proportional to thecross-sectional area of the film, reducing the width of the film alsoreduces the induced voltage. As a result, the readout signal is easilyaffected by noise.

Second, in existing magnetic films, the magnetic moment has a preferredin-plane direction. Thus the device is complicated by the necessity ofapplying currents of different amplitude for storing and retrieving datain the selected orientations. In addition, the thin film devices are notsufficiently small to achieve high densities.

In comparison to magnetic core and thin film memories, semiconductormemory is faster, consumes less power, and can have higher storagedensities. Typical semiconductor memories include Dynamic Random AccessMemory (DRAM), Static Random Access Memory (SRAM), and Read Only Memory(ROM).

DRAM offers relatively high speed, high density, low power consumption,and is readable and writable. However, both DRAM and SRAM are volatile,that is, they lose the stored information when the power is turned off.In addition, DRAM requires a constant refresh of the stored data whichnecessitates complex circuitry. While SRAM does not require a refresh,it has high power consumption and does not have high storage density.

ROM's are non-volatile but the information stored in a ROM cannot beupdated, i.e., data cannot be easily written into a ROM.

In a typical disk storage system, ferromagnetic material having asubstantially square BH loop is coated on the disk; and a magnetic headreads and writes information on the disk as it rotates past the head.The disk is divided into circular tracks. Each track is further dividedinto small regions in which a magnetic moment has two states thatrepresent binary values. An external magnetic field introduced by theread/write head changes the magnetic moment of each small region so asto store a binary value in the region. Thus, to write data, the magnetichead magnetizes an adjacent small region of the rotating disk material.Stored data is retrieved in the form of a voltage induced in the head bythe magnetic moment of the small region as it moves past the head.

Magnetic disk storage systems can store high volumes of data, e.g., 500Megabytes or more. The magnetic disk storage systems, however, are notrandom accessible, operate at slow speed due to the requirement ofmechanical movement, and require complex mechanical and electronicassemblies.

As will be apparent, none of the above-described memory technologiesprovides all the features that are desirable in a memory storage system.Thus, there is a present need to develop a non-volatile, high speed,high capacity, random accessible, static, and updatable storage system.

SUMMARY OF THE INVENTION

The present invention relates to a new composition of materials whichhas ferromagnetic, piezoelectric and electro-optical properties and canbe employed as a storage media. This invention also relates to anon-volatile random accessible memory built on the basis of the inventedcomposition of materials. A novel method for storing and retrieving twoindependent bits of information in a single memory cell of the presentinvention is also disclosed.

The preferred composition of materials of the present inventioncomprises layers of Pb.sub.(1-x-y) Cd_(x) Si_(y), Se.sub.(1-z) S_(z),and Fe.sub.(1-w) Cr_(w) where x, y, z and w indicate the proportions ofthe elements within their respective layers. These values are preferablywithin the following ranges: 0.09≦x≦0.11, 0.09≦y≦0.11, 0.09≦z≦0.11, and0.22≦w≦0.36. In the preferred embodiment, the layers of the compositionof materials also contain the following elements: Bi, Ag, O and N. Theseelements are introduced by electrolysis in a solution containing Bi₂ O₃and AgNO₃.

In a memory device of the present invention, two sets of paralleladdress lines are disposed orthogonally on the opposite sides of aplanar substrate. The layers of the novel composition of materials, asdescribed above, are disposed on both sides of the substrate above theaddress lines with the FeCr layers outermost, and an electrode isconnected to the outermost FeCr layer on each side of the substrate. Anindividual memory cell is located at each crossing point of the addresslines of the two sets.

By applying appropriate current pulses to the two address lines, twoindependent bits of information can be magnetically stored in a singlememory cell. This information is retrieved as a piezoelectric voltagebetween the electrodes that is generated in response to appropriatecurrent pulses applied to the two address lines.

More specifically, to store and retrieve a first bit of information in amemory cell, two synchronized current pulses having the same amplitudeand polarity are applied to two orthogonal address lines. The second bitis stored and retrieved in that memory cell by applying two synchronizedpulses of the same amplitude but opposite polarity to the same twoaddress lines. The current pulses employed for storing binaryinformation are such that the amplitude of a single pulse is notsufficient to alter the state of the stored information but that twoconcurrent pulses are sufficient for storing data. The current pulsesemployed for retrieving the stored binary information have amplitudesinsufficient to alter the stored information.

Such a memory cell is non-volatile, random accessible, static, operatesat high speed, requires low power, is readable and writable, and can bemade in high density arrays.

BRIEF DESCRIPTION

These and other objects, features and advantages of the invention aremore fully set forth in the accompanying Detailed Description in which:

FIG. 1 shows the cross-section of a preferred embodiment of thecomposition of materials of the present invention;

FIG. 2 illustrates the magnetization curve (BH loop) of a conventionalferromagnetic material;

FIG. 3 shows a substantially square BH loop of the composition ofmaterials of the present invention;

FIGS. 4(a)-4(j) illustrate the process of generating piezoelectricvoltage within the composition of materials;

FIGS. 5(a) and 5(b) are the cross section and top-view of the preferredembodiment of the memory device of the invention;

FIGS. 6(a) and 6(b) illustrate the process of selecting carriers withinthe memory device;

FIGS. 7(a) and 7(b) illustrate storing the first bit of information intothe memory device;

FIGS. 8(a) and 8(b) show the process of reading the stored first bit ofinformation;

FIGS. 9(a) and 9(b) illustrate storing a second bit of information intothe memory device;

FIG. 10 shows the current pulses used for retrieving the second bit ofinformation stored in the memory device and corresponding output;

FIG. 11 is a summary list of preferred methods for storing andretrieving information from the memory device; and

FIGS. 12(a) and 12(b) show the electrical current with respect toprocess time in an electrolysis process utilized as a step infabricating the composition of materials.

DETAILED DESCRIPTION

The present invention relates to a composition of materials havingferromagnetic, electro-optic and piezoelectric properties. A randomaccessible non-volatile memory device utilizing the invented compositionof materials is also disclosed. Advantageously, the memory device iscapable of storing two independent bits of information.

The preferred composition of materials comprises layers ofPb.sub.(1-x-y) Cd_(x) Si_(y), Se.sub.(1-z) S_(z), and Fe.sub.(1-w)Cr_(w). The values of x, y, z and w are preferably in the ranges of0.09≦x≦0.11, 0.09≦y≦0.11, 0.09≦z≦0.11, and 0.22≦w≦0.26. Preferably, eachlayer also includes one or more of the elements Bi, Ag, O and N.

Alternatively, in the Pb.sub.(1-x-y) Cd_(x) Si_(y) layer, Ge can beemployed in place of Si and/or Zn or Te can be employed in place of Pb.Also, other conductive elements such as Au, Pt, or Cu can be added tothe layer structure in place of Ag. The invention may also be practicedusing concentrations of Cr in the Fe.sub.(1-w) Cr_(w) layer such that wranges from 0.18 to 0.30.

More specifically, as shown in FIG. 1, a preferred embodiment of theinvented composition of materials comprises a Pb₀.80 Cd₀.10 Si₀.10 layer110, a Se₀.90 S₀.10 layer 120, and a Fe₀.76 Cr₀.24 layer 130. The Fe₀.76Cr₀.24 layer is mainly responsible for the ferromagnetic properties ofthe composition of materials, and the Pd₀.80 Cd₀.10 Si₀.1 and Se₀.9 S₀.1layers are mainly responsible for its electro-optical properties. Allthree layers exhibit piezoelectric properties.

In the devices described below, these layers are sequentially formed ona substrate 100 and each of the Pb₀.80 Cd₀.10 Si₀.10, Se₀.90 S₀.10 andFe₀.76 Cr₀.24 layers is 0.5 μm thick.

Physical properties of the invented composition of materials aredescribed below. Understanding of these properties will help tounderstand the operation of the memory device employing this compositionof materials.

By way of background, a ferromagnetic material exhibits a permanentmagnetic field in the absence of an external magnetic field. Suchmaterials can be described in terms of a large number of small magnetsknown as magnetic dipoles. An external magnetic field applied to aferromagnetic material aligns the magnetic dipoles within the materialin the direction of the applied field, so that the total magnetic fieldwithin the material is the sum of the external field and the fieldgenerated by the aligned magnetic dipoles. When the influence of anexternal magnetic field is discontinued, the orientation of magneticdipoles does not change, resulting in a constant magnetic field in thematerial. Magnetic information storage is based on this property offerromagnetic materials.

FIG. 2 shows an exemplary magnetization curve of a typical ferromagneticmaterial. The magnetization curve is also referred to as a BH loop. They axis in this figure represents magnetic induction B, which is theoverall magnetic field in the material, and the x axis represents themagnetic field strength H of the external magnetic field. Thus, the BHloop shows the change in the magnetic induction B with changing magneticfield strength H.

Let us consider the BH loop of FIG. 2 in further detail. Assuming thatinitially the orientations of magnetic dipoles of the ferromagneticmaterial are evenly distributed in all directions, the total value of Bin the absence of the external field is zero (point "a" on the curve).When an external magnetic field is applied to the ferromagneticmaterial, the value of B gradually increases as H increases until itreaches a point where magnetic induction B begins to saturate (point "b"on the curve). In other words, when H reaches a certain value, B remainssubstantially at B₀ even if H is being increased. If, after saturation,the external magnetic field is decreased to H=0, magnetic fieldinduction B does not return to the point "a" (B=0). Instead, the valueof B remains approximately at B=B₀ (point "c" on the curve).

At point "c" the direction of the external magnetic field H is reversed.At approximately H=-H_(c), the external field changes the polarity ofthe field B, and, at point "e", the field saturates at the oppositepolarity B=-B₀.

Increasing the field strength H causes B to change from point "e" on thecurve to point "b", as illustrated in FIG. 2.

FIG. 3 illustrates the BH loop of the composition of materials of thepresent invention. As in FIG. 2, the x-axis indicates the external fieldstrength H and the y axis indicates the magnetic induction B. It isimportant to note that for the invented composition of the materials,the shape of the BH loop is substantially square with the angle αbetween the y-axis and the BH loop at B=0 being less than 1°. Becausethe magnetization curve is substantially square, the magnetic inductionB is almost invariably at one of two discrete, stable states, +B₀ and-B₀. Accordingly, the novel composition of materials is suitable forstoring binary information.

The composition of materials of the present invention also haspiezoelectric properties. In general, if the mechanical pressure on apiezoelectric material is reduced, a piezoelectric voltage is generated.In the present invention, if the mechanical pressure on the compositionof materials is reduced in a direction substantially perpendicular tothe plane of the layers within the composition of materials, apiezoelectric voltage is generated across the layers. In the presentinvention, the change in mechanical pressure is produced by a change inthe magnetic states of the composition of materials.

An illustrative structure exhibiting the piezoelectric properties of thepresent invention is shown in FIG. 4(a). An explanation of its operationis set forth in conjunction with FIGS. 4(b)-4(j).

FIG. 4(a) illustrates a structure 190 comprising two layers of thecomposition of materials of the present invention. Specifically, thestructure comprises a first FeCr layer 200, a first SeS layer 210, afirst PbCdSi layer 220, a second PbCdSi layer 230, a second SeS layer240, and a second FeCr layer 250. In addition a wire 260 passes throughthe middle of the structure, parallel to the layers.

As shown in FIG. 4(b), an electrical current applied to the wire 260 ina direction that is into the page generates a substantially circularmagnetic field around the wire, as indicated by a circle B_(r) in theclockwise direction indicated by the arrow. Arrows 270 illustrate thedirections of the magnetic dipoles in FeCr layers 200, 250 under theinfluence of this external field. If we divide the structure into twosections 275, 280 that are symmetric about a vertical axis 265perpendicular to wire 260 as shown in FIG. 4(b), the dipole arrangementin sections 275, 280 is equivalent to two magnets of the same strengthhaving north and south poles as indicated by arrows 282 and 284 in FIG.4(c). The length of each arrow represents the amplitude of the magneticinduction B of the corresponding magnet. Due to the attraction betweenthe South pole S and the North pole N of each magnet, the storage mediais mechanically compressed in the direction perpendicular to the layersof the structure.

The BH loop of the magnetic induction B_(r) is shown in FIG. 4(d). Asdescribed previously the BH loop is substantially square, exhibiting twodiscrete, stable magnetic states, +B₀ and -B₀.

Additionally, the magnetic field has a critical field strength H_(c)which is defined as the amplitude of the magnetic field strength whichcauses switching between +B₀ and -B₀. Consequently, if H is greater thanH_(c), the magnetic induction B_(r) will have a value +B₀. If H is lessthan -H_(c), B_(r) will have a value -B₀.

Assume that initially under the influence of the applied external field,the magnetic state is described by point "a" on the curve of FIG. 4(d),where the induction is +B₀. To change the magnetic state of the storagemedia from +B₀ to -B₀, the current through wire 260 must be reduced toreduce the magnetic field strength H. When the current is zero themagnetic strength H is also zero (point "b" on the BH loop). As noted,due to its ferromagnetic properties, even without the external field,the magnetic state of the storage media remains at B₀, i.e., theinformation represented by the magnetic induction B₀ is retained.

When the direction of the current is reversed, the magnetic fieldstrength continues to decrease point. At "c", the magnetic induction Breaches the value B_(c), which is less than B₀. At this point, thedipole moment has been reduced, as shown in FIG. 4(e), because thedipoles begin to realign in the opposite direction. Accordingly, themechanical pressure on the layers due to the attraction of the FeCrlayers 200, 250 has been reduced. The change in the pressure on thelayers causes piezoelectric voltage to be generated perpendicularlyacross the layers. At point "d", where H=-H_(c) and the magneticinduction B is zero, the pressure applied to the layers is minimalbecause the dipoles are aligned in different directions. At this point,the induced piezoelectric voltage reaches its maximum value, due to themaximum change in the pressure on the layers.

As H continues to decrease below -H_(c), the magnetic state switchesfrom point "d" to the point "e" and then to point "f" where it reachesthe second stable state B=-B₀. FIG. 4(f) shows that at point "f" thepoles of the magnets have been reversed. Thus, at point "f", themechanical pressure on the layer returns to its initial value,diminishing the piezoelectric voltage. Increasing the reversed currentfurther (from point "f" to point "g") would not increase the magnitudeof the dipole moment and therefore would not increase mechanicalpressure on the layers.

FIG. 4(g) illustrates the piezoelectric voltage that corresponds tovarious points on the BH loop of FIG. 4(d) as B₀ changes to -B₀. In FIG.4(h), the piezoelectric voltage generated in response to a current pulseis illustrated in the time domain. The piezoelectric voltage is apiezoelectric voltage pulse that is delayed from the time of theapplication of the current pulse. The current pulse applied to the wirehas an amplitude -I that is sufficient to switch B₀ to -B₀,

Similarly, switching from the magnetic state -B₀ to +B₀ generates anegative piezoelectric voltage pulse.

As indicated in FIG. 4(d), a current that generates a field having anamplitude greater than H_(c) is required for switching between themagnetic states. If, however, a current is applied having a lesseramplitude which causes B to assume a value indicated by point "c" inFIG. 4(e), the magnetic state is unstable. In such case, the magneticinduction B tends to oscillate between the values of B₀ (point "b") andB_(c) (point "c"). A piezoelectric voltage pulse generated in responseto such oscillation is shown in FIG. 4(i). The amplitude V₂ of thispiezoelectric voltage pulse is smaller than the amplitude of the pulsegenerated as a result of switching from +B₀ to -B₀ (FIG. 4(g)).

FIG. 4(j) shows a current pulse I which causes B to assume the valueB_(c). The piezoelectric voltage pulse, generated in response to thiscurrent is shown in the lower portion of the figure. The shaded areareflects the oscillation between the two states (B_(c) and +B₀), as itwould be observed on an oscilloscope. As described subsequently, thepiezoelectric voltage generated in response to the current that disturbsbut does not switch the magnetic states, can be employed for readingmagnetically stored information.

FIGS. 5(a) and 5(b) illustrate the cross-section (not to scale) and thetop-view of a preferred embodiment of a portion of a memory device 290of the present invention. The memory device comprises a silicon planarsubstrate 330, first address lines 320 formed on one surface of thesubstrate, and second address lines 340 orthogonal to the first lines,formed on the opposite surface of the substrate. A first set 310 and asecond set 350 of the layers of materials of the present invention aredisposed on opposite sides of the substrate over the address lines.Electrodes 300, 360 are connected to the layers 310, 350 respectively ofthe composition of materials.

The first and second address lines are silver strips approximately 2 μmwide and approximately 1 μm thick. Illustratively the spacing betweenadjacent address lines is approximately between 9 to 20 μm, depending onthe desired density of the memory device. For example in one embodimentthe spacing is 9.5 μm and in a different embodiment the spacing is 19μm. Each set of layers of materials 310, 350 comprise a Pb₀.80 Cd₀.10Si₀.1 layer, a Se₀.90 S₀.10 layer, and a Fe₀.76 Cr₀.24 layersequentially formed on one of the address lines 320, 340 on the Sisubstrate with the two FeCr layers being outermost. Also, each of thelayers is preferably 0.5 μm thick so that each set is preferably 1.5 μm.Each layer is homogeneously saturated with Bi, Ag, O, and N. Preferablythe substrate is 40 μm thick and the electrodes are 1 μm thick silverlayers.

The fabrication of this device begins with depositing 1 μm thick metal(preferably silver) layers onto the opposite surfaces of a 40 μm thicksilicon planar substrate. Alternatively, substrates made of othermaterials such as BaF₂ could be used in place of the Si substrate. Thedeposition is conducted by a conventional technique such as thermalevaporation, e-beam evaporation, or sputtering. The deposited silverlayers are then photolithographically patterned and etched to form aseries of metal strips, each having a width of approximately 2 μm. Theseries of strips on one side of the Si substrate is orthogonal to thestrips on the opposite side. The strips on both sides of the substrateform a cross-barred structure.

The layers of Pb₀.80 Cd₀.10 Si₀.10, Se₀.90 S₀.10, and Fe₀.76 Cr₀.24 arethen deposited sequentially. Prior to the deposition, components of thelayers are prepared by mixing together proper amounts of powder of eachrequired element. The amount of powder of each element corresponds tothe desired proportion of the element in the corresponding layer. Forexample, for the deposition of the Pb₀.80 Cd₀.10 Si₀.10, layer thepowders of Pb, Cd and Si are mixed in the proportions 80:10:10. Afterthe powders of Pb, Cd, and Si are well mixed, the mixture is pressed andbaked to form a suitable source of materials for the selected depositiontechnique. The source materials for deposit of the Se₀.90 S₀.10 andFe₀.76 Cr₀.24 layers are prepared in a similar fashion.

The layers of Pb₀.80 Cd₀.10 Si₀.10, Se₀.9 S₀.1 and Fe₀.76 Cr₀.24 arethen sequentially deposited onto both sides of the substrate. Thedeposition can be accomplished using well-known methods. For example inthe preferred embodiment a plasma sputtering techniques is utilized forcreating the layered structure. After each layer is deposited bysputtering, the temperature of the layer is raised rapidly (i.e. inabout 1.5 seconds) to approximately 500° C. and then cooled toapproximately a room temperature for the deposition of the next layer.As conventionally done, the spattering is performed in vacuum utilizingAr gas. As indicated, each of the Pb₀.80 Cd₀.10 Si₀.10, Se₀.90 S₀.10,and Fe₀.76 Cr₀.24 layers is approximately 0.5 μm thick, forming twostructures approximately 1.5 μm thick on the opposite surfaces of thesubstrate with the two FeCr layers being outermost.

The elements Bi, Ag, O and N by are then added to the layers by anelectrolysis process that employs a heated electrolyte containing Bi₂ O₃and AgNO₃. The electrolyte is prepared by heating high purity water to97° C. in a stainless steel container with a stirring device on thebottom of the container. The Bi₂ O₃ and AgNO₃ powders are then added tothe heated water to form the electrolyte. Preferably the proportion ofweight of the added powders is about 40% of Bi₂ O₃ and 60% of AgNO₃. Theamounts of these powders can be adjusted to achieve a desired current inthe electrolyte. After adding the powders, the electrolyte is maintainedat 97° C. and stirred continuously for at least one hour to form auniform solution.

Prior to the electrolysis process, all the metal strips on both sides ofthe substrate are connected to form a single electrode. The substrate isthen immersed in the electrolyte which is maintained at a temperature of97° C. and is continuously stirred. Advantageously, many substrates canbe simultaneously immersed in the electrolysis solution. For example,100 1 cm×1 cm substrates can be processed simultaneously. In this casethe metal strips of all the substrates should be connected to a singleelectrode.

The complete electrolysis process takes 45 days. Each day the sameprocess cycle is repeated. During the initial 10 hours of the cycle, anelectrical potential of +60 V is applied to the substrates and duringthe next 14 hours, a -60 V potential is applied to the substrates. Thestainless steel container is always kept at ground potential. Also,every 12 hours during the electrolysis process, the positions of thesubstrates within the container are interchanged for uniform processing.Throughout the process the electrolyte is continuously stirred.

During the process, the amplitude of the current flow in the electrolytesolution is monitored. FIG. 12(a) illustrates the current I in Amperesin the electrolyte during the first forty days of the process. The daysare indicated on the horizontal axis "t". FIG. 12(b) illustrates thevalues of the current I during the last five days. The current I is inmA's.

At the end of the forty-fifth day, the electrode is disconnected fromthe substrates and the substrates are removed from the electrolytesolution. At this point, the ions of the elements listed above havesufficiently penetrated the layered structure. Note that in a differentembodiment ion implantation techniques can be employed for introducingthese elements into the layered structure.

The composition of materials on both surfaces of the substrate is thenpolished until the surfaces are substantially smooth. Subsequently, anapproximately 1 μm thick layer of silver is deposited on the surfaces ofeach substrate to form the electrodes 300, 360.

At the completion of this procedure, a novel two-dimensional memoryarray has been manufactured. The area near each intersection of theorthogonal address lines 320, 340 constitutes a cell of the memory.

More specifically, as shown in FIGS. 5(a) and 5(b), the set of metalstrips on the lower surface of the Si substrate forms a first set ofaddressing lines, (referred to as X lines), and the set of metal stripson the upper surface of the substrate forms a second set of addressinglines, (referred to as Y lines). When two electrical currents I_(i) andI_(j) are simultaneously applied to a given line X_(i) of the X linesand a line Y_(j) of the Y lines, respectively, a memory device (i,j) atthe intersection of X_(i) and Y_(j) is selected. By properly choosingthe magnitudes and polarities of the currents I_(i) and I_(j),information can be stored or retrieved from the memory device (i,j).Thus, the memory array, comprising the memory devices of the presentinvention, is random accessible.

The process of storing information in a cell will be apparent from FIGS.6(a), 6(b), 7(c), 7(b), 8(a), 8(b), 9(a) and 9(b), 10 and 11. FIGS. 6(a)and 6(b), show top views of a single cell of the memory device.

The orthogonal address lines 325, 345 divide the cell into four quarters370, 375, 380 and 385, as illustrated in FIG. 6(a). As discussed below,a first bit of information is magnetically stored in the quarters 370and 380 and a second bit is magnetically stored in the quarters 375 and385. For simplicity, the quarters 370 and 380, where a first bit ofinformation is stored, are collectively referred to as carrier "a" andthe quarters 375 and 385 where a second bit is stored are collectivelyreferred to as a carrier "b".

To store a bit of information in one of the carriers of the memorydevice, two electrical currents having specified amplitudes andpolarities are applied to the first and second address lines, 325, 345.Information is retrieved from one of the carriers by applying twoelectrical currents to the address lines and measuring a piezoelectricvoltage generated between the upper and lower electrodes.

The current applied to the first address line is represented as I_(i),and the current applied to the second address line is represented asI_(j). The directions of I_(i) and I_(j) are indicated by the arrowsentering the address lines. In the preferred embodiment, the currentsI_(i) and I_(j) have the same amplitude, I₀. Each current generates aninduced circular magnetic field around the address lines as illustratedby the arrows 390 and 395.

The directions of the magnetic fields B_(i) and B_(j) induced by I_(i)and I_(j) in each quarter is illustrated in FIGS. 6(a) and 6(b). A dot(•) indicates that the field is in the "up" direction and a cross (x)indicates that the field is in the opposite or "down" direction.

As illustrated in FIG. 6(a), in the quarters 485 and 475 (carrier "b")B_(i) and B_(j) have the opposite directions and thus cancel each otherout. For this reason the currents illustrated in FIG. 6(a) do not affectthe information stored in the carrier "b". On the other hand, in thequarters 470 and 480 (carrier "a") the fields B_(i) and B_(j) areinduced in the same direction. Accordingly, these fields enhance eachother and, thus, can alter the stored information.

Thus two currents having the same polarities and amplitudes applied tothe address lines, affect only the magnetic state of the carrier "a" andthereby select this carrier. Likewise, two negative pulses also selectand can store data in the carrier "a". Note also that the amplitudes ofthe currents that select the carrier "a" do not have to be equal, aslong as their combined effect does not change the magnetic state in thecarrier "b".

FIG. 6(b) illustrates the process of selecting the carrier "b". Twocurrents, having opposite polarities, I₁ =+I_(o) and I_(j) =-I_(o), areapplied to the first and second address lines respectively. Asillustrated using the dot and cross convention described above, in thecarrier "a", the fields generated by these currents cancel each otherout, without affecting the magnetic state. In carrier "b" however thefields generated by these currents enhance each other so that thecarrier "b" is selected.

Similarly, two currents I_(i) =-I_(o) and I_(j) =+I_(o) applied to thetwo address lines also select the carrier "b". Thus two currents withthe same amplitude but the opposite polarities select the carrier "b"for storing or retrieving information.

To store information, the amplitudes of the two currents combined shouldbe sufficiently large to switch the magnetization of a carrier betweenthe magnetic states B₀ and -B₀. In addition, the amplitudes of the twocurrents combined should be sufficiently small so that a single currentalone is unable to change the magnetic state of a carrier. This isnecessary to assure that only one carrier in the memory array isselected by the signal on an address line.

To retrieve information, the amplitudes of the two currents combinedshould be small enough that the induced field is not strong enough tochange the magnetic state of the carrier. The combined amplitudes,however, should be sufficient to disturb the magnetic state of thecarrier so as to generate a piezoelectric voltage across the storagemedia. As discussed above, the direction of this piezoelectric voltagerepresents the binary data stored in the carrier.

By way of illustration, FIG. 7(a) depicts the process of writing abinary "1" into the carrier "a" using synchronous current pulses on thetwo address lines. Initially, all the cells of the array are assumed tobe in the "0" state which corresponds to a magnetic induction of -B₀. Towrite a binary "1", two synchronized current pulses, I_(i) =+20 μA andI_(j) =+20 μA are applied to the two address lines, respectively. Thisgenerates a magnetic field H which magnetizes the FeCr layers of thecomposite material. The magnetic induction B_(a) of these layers isdepicted in FIG. 7(a) as a closed loop with an arrow. For the structureof FIG. 5 having the dimensions described above, the amplitude of thecritical current I_(c) necessary to generate the critical field strengthH_(c) required for switching between the two discrete states isapproximately 35 μA. At the cell where the two pulses coincide, the two+20 μA currents create a field H that would be generated by applying a40 μA current. Since this current is greater than I_(c), the magneticinduction becomes B₀ so that a binary "1" is stored. As explainedpreviously, after the pulses have ended, the magnetic induction at thecell B_(a) remains equal to B_(o), so that a binary "1" is retained inthe carrier "a".

As shown in FIG. 7(b), to store a binary "0" into the carrier "a", twosynchronized current pulses, I_(i) =-20 μA and I_(j) =-20 μA are appliedto the address lines respectively. Since the sum of these currents is-40 μA, which is less than -I_(c), the current pulses switch themagnetic state from +B₀ to -B₀.

Switching between +B₀ and -B₀ generates a piezoelectric voltage pulsebetween the first and second electrodes after a delay Δt from the timeof the application of the current pulses. The piezoelectric pulse ispositive for switching from +B₀ to -B₀ and is negative for switchingfrom -B₀ to +B₀. If the magnetic state does not change, no piezoelectricpulse is generated. Accordingly, the generated piezoelectric voltagepulses can be employed to verify that a bit of binary data has beenstored.

To read information stored in the carrier "a" of the memory, twosynchronized current pulses I_(i) =-15 μA and I_(j) =-15 μA are appliedto the address lines. Since the critical current I_(c) =-35 μA, the -30μA sum of these currents cannot switch the magnetic states from +B₀ to-B₀. This current, however, is sufficient to disturb the magnetic statewithout complete switching. As shown in FIG. 8(a), assuming a binary "1"is stored in the carrier, the applied current pulses change B_(a) from avalue corresponding to a point such as "a" on the magnetization curve tothe value corresponding to point "b" on the curve. Because of thepreviously discussed piezoelectric properties of the cell, this changein the magnetic induction generates a positive piezoelectric voltage ofapproximately +15 μV, indicating a "1" is stored in the carrier.

If a "0" is stored in the carrier "a", the applied current pulses willchange B_(a) from a value corresponding to point "c" to the valuecorresponding to point "d" on the curve. In this case, however, themagnetic induction of carrier "a" remains at B_(a) =-B₀, so that nopiezoelectric voltage is generated, indicating a "0" is stored. Theinformation stored in the device is not changed during the readingprocess because I is less than I_(c).

This process of retrieving data is illustrated in the time domain inFIG. 8(b). The delay between the piezoelectric voltage pulse and thesynchronized current pulses is approximately 0.75 ns.

The storage and retrieving of data for carrier "b" is similar. As shownin FIG. 9(a), two synchronized current pulses, I_(i) =-20 μA and I_(j)=+20 μA are applied to the address line to store a "1" in the carrier"b". As discussed above, such current pulses do not affect the carrier"a". At the point where the pulses coincide, a field is induced that isequivalent to the field induced by the 40 μA current. Since this isgreater than I_(c) =35 μA, a "1" is stored in the carrier "b". As shownin FIG. 10(b), the current pulses I_(i) =+20 μA and I_(j) =-20 μA areapplied to store a "0" in the carrier "b".

Switching from "1" to "0" in the carrier B generates a negativepiezoelectric voltage pulse between the electrodes at a delayed time Δt,and switching from "0" to "1" generates a positive piezoelectric voltagepulse. If the state does not change, no piezoelectric voltage isgenerated.

The data stored in the carrier "b" is retrieved in a similar way asdiscussed in conjunction with carrier "a". As shown in FIG. 10, toretrieve data stored in the carrier B, two synchronized current pulses,I_(i) =+15 μA and I_(j) =-15 μA are applied. The combined magnitudes ofthese pulses are not large enough to switch the magnetic state incarrier "b". If a "0" is stored, the current pulses would not change themagnetic induction B_(b) of carrier "b", and no piezoelectric voltagebetween the electrodes is generated. If a "1" is stored, the appliedcurrent pulses would disturb the magnetic state B_(b) =B₀, but would notchange it, generating a positive piezoelectric voltage pulse at a delayΔt. Thus, for the carrier "b", no piezoelectric voltage pulse indicatesthat a "0" is stored, and a positive piezoelectric voltage indicatesthat a "1" is stored.

FIG. 11 summarizes the above-described methods of storing and retrievingdata from the carriers "a" and "b" of the memory device.

Different methods of storing and retrieving information from the memorydevice of this invention can also be employed. For example, twosynchronized currents I_(i) =+15 μA and I_(j) =+15μ can be utilized forretrieving information from the carrier "a". Similarly, two synchronizedcurrents I_(i) =-15 μA and I_(j) =+15 μA can be used to retrieve datafrom the carrier "b". A method of destructive readout can also beemployed. For example, two synchronized current pulses I_(i) =+20 μA andI_(j) =+20 μA, that write a "1" in the carrier "a", can be applied andthe piezoelectric voltage generated in response to these pulses wouldidentify the previously stored data, thereby destructively retrievingdata from the carrier A.

One of the advantages of the memory device of the present invention isits low power consumption as compared with the prior art non-volatilemagnetic memory devices. Since the storage media employed in this deviceis highly sensitive to the magnetic field generated by the drivingcurrents, it can quickly switch between "0" and "1" at relatively smalldriving currents, about 20 μA on each line. Consequently, the powerconsumption is low for storing and retrieving data. In one embodiment,it consumes approximately 3.4×10⁻¹⁰ w for reading and 6×10⁻¹⁰ w forstoring a bit of data into the device.

Note also that the retrieval of information as a piezoelectric voltagegenerated between the sensing electrode is intrinsically faster thangenerating induced electromagnetic voltage as in prior art magneticmemory devices.

Typically, the delay between the current pulses and correspondingpiezoelectric voltage is in the range of subnanoseconds. Switchingbetween "1" and "0" usually takes a few nanoseconds.

Thus a memory device which is random accessible, non-volatile, andoperates in static mode has been described. This memory device offershigh-speed operation, low power consumption, and can store informationat high density.

The claims which follow are to be interpreted to cover all theequivalent structures and methods. The invention is, thus, not to belimited by the above exemplary disclosure, but only by the followingclaims.

What is claimed is:
 1. A composition of materials comprising:a firstlayer of material having a composition of M.sub.(1-x-y) Cd_(x) R_(y)where M is an element selected from the group consisting of Pb, Zn, andTe, and R is an element selected from the group consisting of Si and Ge,wherein x and y are values within the ranges of 0≦x≦1, 0≦y≦1, and0≦(x+y)≦1; a second layer of Se.sub.(1-z) S_(z) formed on the firstlayer, wherein z is a value within the range of 0≦z≦1; and a third layerof Fe.sub.(1-w) Cr_(w) formed on the second layer, wherein w is a valuewithin the range of 0≦w≦1.
 2. The composition of materials of claim 1wherein the first layer further includes at least one of the followingelements: Bi, O, N, and an element selected from the group consisting ofAg, Au, Pt, and Cu.
 3. The composition of materials of claim 2 whereinthe second layer further includes at least one of the followingelements: Bi, O, N, and an element selected from the group consisting ofAg, Au, Pt and Cu.
 4. The composition of materials of claim 3 whereinthe third layer further includes at least one of the following elements:Bi, O, N, and an element selected from the group consisting of Ag, Au,Pt, and Cu.
 5. The composition of materials of claim 1 wherein the valueof x is within the range of 0.09≦x≦0.11.
 6. The composition of materialsof claim 5 wherein the value of y is within the range of 0.09≦y≦0.11. 7.The composition of materials of claim 6 wherein the value of z is withinthe range of 0.09≦z≦0.11.
 8. The composition of materials of claim 7wherein the value of w is within the range of 0.18≦w≦0.30.
 9. Thecomposition of materials of claim 8 wherein the value of w is within therange of 0.22≦w≦0.26.
 10. The composition of materials of claim 1wherein the value of x is substantially 0.10.
 11. The composition ofmaterials of claim 10 wherein the value of y is substantially 0.10. 12.The composition of materials of claim 11 wherein the value of z issubstantially 0.10.
 13. The composition of materials of claim 12 whereinthe value of w is substantially 0.24.
 14. The composition of materialsof claim 13 wherein the first layer is substantially 0.5 μm thick. 15.The composition of materials of claim 14 wherein the second layer issubstantially 0.5 μm thick.
 16. The composition of materials of claim 15wherein the third layer is substantially 0.5 μm thick.
 17. A method ofmanufacturing a composition of materials having ferromagnetic,electro-optic and piezoelectric properties comprising the stepsof:forming a first layer of material having a composition ofM.sub.(1-x-y) Cd_(x) R_(y) where M is an element selected from the groupconsisting of Pb, Zn, and Te, and R is an element selected from thegroup consisting of Si and Ge, wherein x and y are values within theranges of 0≦x≦1, 0≦y≦1, and 0≦(x+y)≦1; forming a second layer ofSe.sub.(1-z) S_(z) on the first layer, wherein z is a value within therange of 0≦z≦1; and forming a third layer of Fe.sub.(1-w) Cr_(w) on thesecond layer, wherein w is a value of 0≦w≦1.
 18. The method of claim 17further comprising the step of adding at least one of the elements ofBi, O, N, and an element selected from the group consisting of Ag, Au,Pt, and Cu, into the first, second, and third layers.
 19. The method ofclaim 17 further comprising the step of adding at least one of theelements of Bi, O, N, or Ag, into the first, second, and third layers.20. The method of claim 19 wherein the step of adding at least one ofthe elements of Bi, O, N, Ag comprises immersing the composition ofmaterials in an electrolyte containing Bi₂ O₃ and AgNO₃ and performingan electrolysis.
 21. The method of claim 20 wherein the electrolyte issaturated with AgNO₃.
 22. The method of claim 20 further comprising thestep of continuously stirring the electrolyte.
 23. The method of claim22 further comprising the step of heating the electrolyte.
 24. Themethod of claim 23 further comprising the step of maintaining theelectrolyte at substantially 97° C.
 25. The method of claim 20 furthercomprising the steps of applying a negative electrical potential to thecomposition of materials with respect to the electrolyte.
 26. The methodof claim 20 further comprising the steps of alternatively applyingpositive and negative electrical potentials to the composition ofmaterials with respect to the electrolyte.
 27. The method of claim 26wherein, in a 24 hour interval, the negative potential is applied forsubstantially 14 hours and the positive potential is applied forsubstantially 10 hours.
 28. The method of claim 20 wherein theelectrolysis is performed for substantially 45 days.
 29. The method ofclaim 17 wherein the value of x is within the ranges of 0.09≦x≦0.11. 30.The method of claim 29 wherein the value of y is within the ranges of0.09≦y≦0.11.
 31. The method of claim 30 wherein the value of z is withinthe ranges of 0.09≦z≦0.11.
 32. The method of claim 31 wherein the valueof w is within the ranges of 0.22≦w≦0.26.
 33. The method of claim 17wherein the value of x is substantially 0.10.
 34. The method of claim 33wherein the value of y is substantially 0.10.
 35. The method of claim 34wherein the value of z is substantially 0.10.
 36. The method of claim 35wherein the value of w is substantially 0.24.